The present invention relates generally to signal processing systems and more particularly to a method and apparatus for processing a series of digital signals representing a stroke of a writing instrument such as a stylus on a writing surface such as a digitizing tablet to minimize noise at the ends of the stroke.
In many applications a digitizing tablet is used to convert the pen or stylus motion into a set of electrical data which is then processed by digital equipment. Typically, there is a special electronic or electromagnetic surface, which detects the x and y positions of the pen as it moves along the surface at a periodic rate. The information is present as two digital data words at a periodic clock rate. This class of technology is used for signature verifications, automatic drafting, character recognition and so forth. In each case, the user writes on the writing surface with the writing instrument and the position is monitored electronically.
In connecting such a device directly to a processing system there are problems because the raw data contains certain kinds of noise or other defects which can adversely affect applications that process the hand drawn data for editing or character recognition or that produce electronic drafting drawings. The noise may be electrical or mechanical noise produced by the equipment employed to generate the sequence of signals corresponding to the stroke of the writing instrument. The noise may also be natural noise produced by the writer when he is writing slowly relative to the rate at which data is sampled or when he pauses or halts at the ends of the stroke. This latter type of noise is often referred to as hand wobble.
In U.S. Pat. No. 4,375,081 to B. Blesser there is described a technique for removing or minimizing noise over the length of a stroke caused by writing slowly. The technique comprises filtering a signal represented by a first series of indicia wherein each indicium of the series represents an amplitude by serially averaging the amplitudes of sets of n indicia of the series to form a second series of indicia and serially amplitude comparing each subsequently occurring indicium of the second series with a previously occurring indicium thereof to form a third series of indicia which includes those indicia resulting from comparisons having an amplitude difference greater than a predetermined amount.
In U.S. Pat. No. 4,284,975 to K. Odaka there is disclosed a pattern recognition system for hand-written characters operating on an on-line basis comprising a character input unit for providing the coordinates of a plurality of points on strokes of a hand-written input character, an approximate unit for providing some feature points for each stroke of the input character, a pattern difference calculator for providing the sum of the length between the feature points of the input character and those of the reference characters which are stored in the reference pattern storage, and a minimum difference detector for determining the minimum value of the difference among the pattern differences thus calculated and determining the input character as the reference character which provides the minimum difference.
It is an object of this invention to provide a new and improved technique for processing a series of signals representing a stroke of a writing instrument on a surface to remove or minimize noise at the start and finish of the stroke.
It is another object of this invention to provide a new and improved technique for processing a series of signals representing a stroke of a writing instrument on a surface to remove or minimize at the start and finish of the stroke caused by wobble.
It is a still another object of this invention to provide a new and improved technique for processing a series of signals representing a stroke of a writing instrument on a surface to remove or minimize noise at the start and finish of the stroke which does not involve averaging.
It is yet still another object of this invention to provide a new and improved technique for processing a series of signals representing a stroke of a writing instrument on a surface to remove or minimize noise at the start and finish of the stroke which is not dependent on the rate at which the signals representing points along the stroke are sampled and does not require that the points be uniformly spaced from one another.
It is a further object of the invention to provide a technique as described above which does not alter the signals between the points determined to be the end points.